The present invention generally relates to photomasks and photolithography methods using photomasks, and more particularly to a reflection type photomask and a reflection type photolithography method which uses the reflection type photomask.
In the field of semiconductor devices, there is a demand to form finer patterns in order to further reduce the size of integrated circuits. In order to form finer patterns, it is necessary to improve the resolution. The resolution is inversely proportional to the numerical aperture and is proportional to the wavelength of the light which is used for the exposure. However, there is a limit to increasing the numerical aperture, and thus, it is necessary to reduce the wavelength of the light in order to improve the resolution. But on the other hand, various restrictions are introduced when the wavelength of the light is reduced.
According to the conventional photolithography technique, a transmission type mask is transferred on a photoresist layer by use of a transmission type lens when forming a pattern. But when the wavelength of the light is reduced in order to improve the resolution, various restrictions are introduced by the material used for the lens and the substrate material of the mask. That is, when the wavelength of the light is gradually reduced in an ultraviolet region, the electron transition region of the material starts and the light transmittance decreases rapidly. In addition, color center or the like is generated by impurities and lattice defect. Furthermore, it is virtually impossible to use a suitable light transmitting material in the far ultraviolet region.
Therefore, according to the conventional transmission type photolithography technique, it is in actual practice impossible use light having a wavelength of approximately 180 nm or less due to the restrictions posed by the transparent material. As a result, the resolution limit of the conventional transmission type photolithography technique is approximately 0.2 to 0.25 .mu.m.